This invention relates generally to a molten glass forehearth of the type used between a glass melting tank or furnace and a feeder bowl, and more particularly relates to a forehearth having improved heating and cooling characteristics.
In the production of glass, molten glass is produced in a glass melting furnace and then passes along a forehearth in a continuous stream to a feeder bowl from which the molten glass is fed in mold charges or gobs into a glassware forming machine. A typical forehearth comprises a refractory trough along which the molten glass flows and which is provided with an insulating roof.
It is known that the temperature of the glass is not homogeneous throughout its cross-section. The glass tends to be cooler at the outside edges and hotter in the central portion due to the cooling effect of the sidewall of the channel. For this reason, heating means such as gas burners, submersible electrodes, or the like are provided in the side of the forehearth to heat the glass. Cooling air may also be blown into the forehearth either transversely or longitudinally of the direction of gas flow. With the proper rates of heating and cooling, homogeneity of the glass temperature across the stream of glass can be improved.
One such arrangement is shown in U.S. Pat. No. 3,999,972, issued Dec. 28, 1976. According to that patent, the forehearth includes a roof having a plurality of longitudinal ridges depending therefrom which define a central longitudinal channel. Heaters are provided in the sidewall of the roof structure to heat portions of the glass stream at the longitudinal edges of the stream and cooling air flows between inlet and outlet ports in the roof over the central portion of the stream of glass in contact with the upper surface of the stream.
However, when there is direct contact of the glass stream with the cooling fluid, there is a possibility that if the glass is cooled too much, a skin forms on the surface of the glass which is in contact with the cooling air. This skin acts as an insulator retaining the heat within the glass beneath it with the result that the glass cannot be properly cooled.